The overall goal of this research project is to identify amalgam microstructures which optimize clinically important mechanical properties. Diverse microstructures can be produced by controlling the speed of the amalgamation reaction. Consequently, this project has two interrelated major objectives. The first is to determine what factors influence the kinetics of amalgamation of dental amalgams (and therefore, their microstructures). The second is to determine the effect of specific microstructures on amalgam mechanical properties. The research will focus on the combinations of microstructures and mechanical properties characteristic of high copper amalgams which resist marginal fracture in clinical studies. Experiments will determine the effect of composition and microstructures of Ag-Sn and Ag-Sn-Cu alloys on nucleation and growth rates of amalgam phases. These rates will be measured in situ in a scanning electron microscope (SEM). Other experiments will investigate the influence of alloy particle microstructure on the volume fraction, particle size, and distribution of eta Cu-Sn crystals which form in amalgams. Alloy particles with different microstructures will be produced by heat treating atomized Ag-Sn-Cu powders. Two diffusion-controlled transformations, gamma2Sn-Hg to Cu-Sn and gamma1Ag-Hg to beta1Ag-Hg, will be studied. The progress of these transformations with time will be followed in the SEM. Phase changes will be detected with an energy dispersive x-ray analyzer. The very high one-hour compressive strengths of some high copper amalgams will be investigated. The influence of amalgam microstructure on this strength will be determined by analyzing stereo-pair electron micrographs of fractured surfaces. The restriction of grain boundary sliding (GBS) by eta-containing reaction layers will be studied. Using the SEM, GBS next to several reaction layers will be examined at selected times during creep tests. Such restriction of GBS may reduce the creep rates of some high copper amalgams.